Bruguiere, now retired at 65, also describes Pakistan as the "cauldron" of terrorism and blames ex-president Pervez Musharraf for failing to address the threat of extremists during his seven years in power.

"Japan has become a prime target for Al-Qaeda," Bruguiere told journalists late Thursday at an event for the release of his book "Ce que je n'ai pas pu dire" (What I Couldn't Reveal).

Japan's geographical position and, in particular, its large Pakistani community make the country vulnerable to an attack, he said.

The retired judge said he had alerted Japan of the threat as he had done for the United States before the September 11, 2001 attacks.

"The danger nowadays is Pakistan," he said, describing the country as a "cauldron" of extremism that was allowed to fester under Musharraf's rule.

A 2003 investigation found Pakistani army officers were acting as instructors in training camps run by the Islamist group Lashkar-e-Taiba, he said.